1. Field of the Invention
The invention concerns a circuit for the automatic control of the off-load voltage of a device having a load formed by a current source. It can be applied, notably, to differential comparators in which two input signals are compared by two transistors, each loaded by a current source which is itself formed by a transistor, the gate and source of which are connected. The circuit according to the invention further provides, in this case, for the self-calibration, namely the compensation of offset between the two arms of the comparator.
2. Description of the Prior Art
It is known that a transistor having its source and its gate connected together (Vgs=0) forms a current source which delivers a constant current proportionate to the size of the transistor. However, owing to the very small sizes (in the range of micrometers) of present-day transistors, and especially in the microwave field, it is not always possible to obtain a plurality of transistors which are strictly identical to one another. As a consequence, two identically designed current sources may be made in a manner that is not strictly identical for reasons of technology.
Thus, if we consider a known simple differential comparator, such as the one shown in FIG. 1: it has two transistors T2 and T3, the gates of which form the inputs E1 and E2 of the comparator. These two transistors are powered by a current source T1 and loaded by two transistors, T4 and T5, which are mounted as current sources and have outputs S1 and S2 made on their gates. If, for technological reasons, the current set by the current source is not strictly equal to that set by the current sources T4 and T5, the result thereof is a major difference between the mean off-load levels at S1 and S2. The term "mean off-load levels" refers to the voltages obtained when the input signals at E1 and E2 are equal and in the middle of the dynamic range.
By way of example, a difference of 10% on the gate width of T4 or T5 takes the off-load level to saturation, which makes the use of this comparator very difficult. It is therefore necessary to stabilize this comparator, especially if it works in microwave mode, hence with sub-micronic gate widths.
Furthermore, if the current set by T4 is not strictly equal to that set by T5, the off-load levels of S1 and S2 are not the same a so-called offset voltage appears.
There is a known stabilization device such as the one shown in FIG. 2. A control loop is added to a comparator such as the one of FIG. 1. This control loop has as follower transistor T9, a plurality of diodes D1 . . . D3 . . . and a current source T11. The output signal S1 is applied to the gate of the transistor T9 and the voltage tapped between the diodes and the current source T11 is applied to the gate of the current source T1. This circuit controls the off-load level S2 by adjusting the current T1.
However, while a device such as this is satisfactory in certain cases, it has a few imperfections:
the amplitude of the output signals at S2 is twice as great as the amplitude at S1 for the circuit is not symmetrical; PA1 the shifter circuit needed at the stabilization loop, namely the row of diodes, has very poor behavior under temperature; PA1 this system achieves no self-calibration: the offset between S1 and S2 is not compensated for. PA1 the fraction common to the two variable voltages of the two pseudo-generators stabilizes the mean off-load levels of the outputs S1 and S2 PA1 the difference between these two voltages compensates for the offset between the outputs S1 and S2. PA1 means for the automatic control of the gate/source voltage Vgs of said transistor, these means being formed by a follower-shifter circuit which acts as a voltage pseudo-generator, PA1 means to control the shift voltage of said follower-shifter circuit by the load stored in a capacitor; PA1 means for charging said capacitor by a shifter circuit which, during a calibration stage, taps the output voltage from the device and reinjects it into the storage capacitor.